1. Field of the Invention
This invention relates to a process for treating a semiconductor substrate, particularly to a process which can show an excellent effect of removing metallic impurities and reduce deposit of microparticles.
2. Description of the Prior Art
Such processes for treating semiconductor substrates conventionally used include H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 (hereinafter referred to as SPM) or NH.sub.4 OH/H.sub.2 O.sub.2 /H.sub.2 O (hereinafter referred to as APM) treatment for removing organic matters; SPM or HCl/H.sub.2 O.sub.2 /H.sub.2 O (hereinafter referred to as HPM) treatment for removing metallic impurities; and a buffered hydrofluoric acid (hereinafter referred to as BHF) or dilute hydrofluoric acid (HF/H.sub.2 O hereinafter referred to as DHF) treatment for removing oxide films. These processes all predominantly employ a so-called dipping treatment where semiconductor substrates are treated by dipping them in a treating solution. In addition to the above processes, a dry method utilizing ozone for removing organic matters or utilizing hydrofluoric acid vapor for removing oxide films are gradually put into practical uses.
The dip treatment in these conventional processes for treating semiconductor substrates employs one or more treatments using treating solutions selected from APM, SPM, HPM, DHF and BHF. However, APM shows relatively great effect of removing metals which are liable to form complexes with ammonia such as Cu but generally has poor ability of removing single metals compared with the treating solutions based on acids such as SPM and HPM. Further, while hydrogen peroxide is added as an oxidizing agent to APM so as to prevent overetching of semiconductor substrates, any metallic impurities very susceptible to oxidation such as Al and Fe contained in the treating solution will be taken or included into the natural oxide film to be formed during the treatment due to the high oxidation property of hydrogen peroxide. The metal impurities thus taken into the oxide film diffuse throughout the semiconductor substrate during the subsequent heat treatment to cause increase in the junction leakage, reduction of carrier lifetime and deterioration of the property of holding DRAM. Moreover, when gates are formed, such metal impurities diffused cause reduction in the withstand voltage of the gate oxide film or Vt fluctuation. Accordingly, APM is scarcely used singly in a washing process for removing such metal impurities, and an additional acid treatment such as with SPM or HPM, or a DHF or BHF treatment for removing the natural oxide film to be formed during the APM treatment is carried out.
SPM and HPM have a considerably great effect of removing metallic impurities since they can remove such impurities by dissolving them therein with the aid of the acid components. However, even SPM or HPM causes a natural oxide film to be formed on the substrate surface by the act of the hydrogen peroxide contained in the treating solution, and when the concentration of the metallic impurities in the treating solution is high, these impurities are taken into the natural oxide film.
As more and more integration proceeds in devices, recently the gate oxide film and the capacitative insulating film are made thinner and thinner. Treatment with APM, SPM or HPM inevitably forms a natural oxide film having a thickness of 10 to 20 .ANG.. When a thin oxide film with a thickness of about 100 .ANG. is to be formed, the natural oxide film causes deterioration of the oxide film or nonuniform film thickness. Accordingly, higher importance is put on the prevention of such natural oxide film formation.
On the other hand, inclusion of metallic impurities such as Al and Fe hardly occurs by the treatment with DHF or BHF since DHF and BHF can remove the natural oxide film. The DHP or BHF treatment, however, suffers a problem that any metal particles having less ionization tendency (e.g. Cu and Au) than in silicon present in the treating solution cause to expose the silicon crystal surface, so that the metal particles precipitate on the substrate surface.
The treatment with DHF or BHF further suffers a problem that microparticles deposit on the semiconductor substrate. It is said that the size of the microparticles which affect the characteristics of devices is about 1/10 of the size of the design rule. For example, in a 4M DRAM, microparticles with a particle size of 0.1 .mu.m or less affect the device characteristics. The microparticles deposited on the semiconductor substrate cause, for example, pattern defects in lithography or abnormal thickness of the oxide film in an oxidation.diffusion process, leading to deterioration in the withstand voltage of the gate oxide film or abnormal diffusion.
On the other hand, dry washing typified by UV-ozone washing and hydrofluoric acid vapor washing scarcely causes in principle deposition of microparticles or contamination with metallic impurities, but the metallic impurities originally deposited on the semiconductor substrate can hardly be removed therewith.
As described above, none of the conventional treating process can show excellent effect of removing any metals with reduced number of deposited microparticles.